An automotive heating, ventilation, and air conditioning (HVAC) system usually comprises an evaporator for cooling air and a heater-core for heating air. The heater-core is designed to transfer heat from the coolant of a vehicle's engine to the passenger compartment air in order to warm the passengers and defog or de-ice the windows of the vehicle. The warm air flow from the heater-core is mixed with a cold air flow from the evaporator and discharged through vents into the compartment of the vehicle.
The temperature of the discharged air can be controlled by various systems. One kind of these systems applies an open loop algorithm in order to control the position of blend doors, which control the air flows in the HVAC system and are located in air ducts of the HVAC system. In such open loop control systems, data about the temperature of the air flow leaving the heater-core (in the following also called heater-core air discharge temperature) and or the evaporator (in the following also called evaporator air discharge temperature) is important in order to estimate the air discharge temperature of the HVAC system.
In existing systems, the heater-core air discharge temperature is e.g. obtained by a direct temperature measurement using a temperature sensor. However, a temperature sensor increases the overall costs of the HVAC system. According to another solution, the heater-core air discharge temperature is roughly estimated based on the temperature of the coolant. This solution is shown in FIG. 3. The coolant temperature 100 is supplied to a low pass filter 102 as a measurement signal, preferably on a CAN bus. The low pass filter 102 maps the heater-core air discharge temperature 104 in function of the coolant temperature. This solution has the disadvantage that the accuracy of calibration over a wide range of operating conditions of the HVAC system is poor, particularly since the heater-core air discharge temperature depends on a plurality of parameters and not only on the coolant temperature.